http: stop dropping large custom headers

[GrahamCampbell]

Custom request headers larger than the response-header parser limit have been silently omitted or misinterpreted since curl 8.13.0 because outbound parsing treats a bounded-parser failure as an empty field. This restores delimiter-based parsing for custom fields and token checks, transmits fields that fit each destination request buffer, and returns CURLE_TOO_LARGE when a request cannot fit.

[Copilot]

Pull request overview

This PR fixes HTTP custom request header handling so that very large user-provided headers are no longer silently dropped/misparsed due to reuse of the response-header parser size limit, and instead are correctly parsed/sent up to the request buffer limit (failing with CURLE_TOO_LARGE when the request cannot fit).

Changes:

• Switch header value/token parsing in lib/http.c from bounded “response header size” parsing to delimiter-based parsing for custom/outbound header handling.
• Ensure blank custom headers ("Name;" form) are appended without integer truncation risks for long header names.
• Add regression tests covering: (1) large custom headers that should be transmitted, and (2) oversized custom headers that must fail with CURLE_TOO_LARGE.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/http.c	Adjusts header parsing and custom header emission to handle large outbound headers correctly and fail when exceeding request buffer limits.
tests/unit/unit1625.c	Extends unit coverage for Curl_compareheader() to succeed with very large header values.
tests/data/test3228	New HTTP test verifying large custom request headers (over response-header limit) are still transmitted.
tests/data/test3229	New HTTP test verifying an oversized custom request header fails with CURLE_TOO_LARGE.
tests/data/Makefile.am	Registers the new test cases in the test suite.

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